I take magnesium for three reasons - migraines, endurance and bone health. Magnesium is vital to life, being part of every cell. It is even a critical part of energy supply, being needed for ATP usage. I believe it is a critical micronutrient for runners.
Only 32% of the US population gets their RDA of magnesium, and only 27% in NC  Magnesium levels in the blood fall after a marathon, which suggests that runners are more at risk for magnesium deficiency than sedentary people. There is even some suggestion that magnesium deficiency is related to the death of a marathon runner due to mitral valve prolapse.
There is some evidence for magnesium supplementation for improving athletic performance. "That means that athletes wouldn't be able to work or train as long as they would if they had better magnesium levels" . There is evidence that magnesium deficiency can result in a significant reduction in exercise performance . There is increased loss of magnesium in athletes  through sweat and urine. It seems a reasonable assumption to this author that the increased sweating that occurs in an NC summer would exacerbate this loss.
4 Bone Health
Magnesium is a critical component of bones, making up about 1% of the structure. Lower levels of magnesium produce, more brittle bones with larger bone crystals . Inadequate magnesium results in lower blood calcium levels, resistance to hormone that control bone density and reduction of vitamin D effect; all result in reduced bone density . There is some evidence that magnesium supplementation on its own will help bone density . One study showed this in healthy older white subject, but not in black subjects . Magnesium is also believed to be critical to calcium absorption . (If you have concerns over bone density, don't forget to make sure you are getting enough Vitamin D.)
There is evidence for magnesium supplementation helping prevent migraines . Personally I have seen a reduction in my migraines, and an improvement in my low light vision. If you suffer from migraines, I would strongly recommend reading the reference articles. I also found that the supplement MSM caused me migraines, but vitamin B2 has reduced them.
7 Other Impacts of Deficiency
There are studies that have linked magnesium deficiency with asthma, emphysema, Attention deficit/hyperactivity disorder (ADHD), Depression, type 2 diabetes, heart disease, high blood pressure in women, and several other problems. 
8 Increasing Intake
Magnesium, is absorbed at about 30-40% efficiency . Good sources of magnesium in food include nuts, cereals, coffee, green leafy vegetables and especially chocolate . Cheap supplements use magnesium oxide, which is very hard to absorb; I use magnesium orotate, though chelated forms are also supposed to work well. Avoid taking Magnesium sulfate as it is used to treat constipation . Vitamin C can help absorption of minerals. Therefore I take magnesium with Vitamin C on an empty stomach. (Bathing in Epson Salts can also raise magnesium levels in the blood.)
My research indicates that magnesium is generally a safe supplement at RDA type levels. Excess magnesium is filtered by the kidneys, so overdose is normally only an issue for people with kidney issues. Magnesium supplements can cause diarrhea, but this seems to be linked to the type of supplement. As with all my advice, verify it independently. If you have any doubts, consult your physician.
10 My Usage
I buy my Magnesium Orotate from Lucky Vitamin as they seem to have the best price I have found - http:://www.luckyvitamin.com/item/itemKey/55469. I generally take one tablet (500mg) with Vitamin C on rising, and another post run.
- ↑ Magnesium deficiency (medicine) http:://en.wikipedia.org/wiki/Magnesium_deficiency_%28medicine%29 http://en.wikipedia.org/wiki/Magnesium_deficiency_%28medicine%29
- ↑ USDA intake figures for NC http:://www.ars.usda.gov/Services/docs.htm?docid=11198 http://www.ars.usda.gov/Services/docs.htm?docid=11198
- ↑ The dangers of magnesium deficiency in endurance athletes http:://findarticles.com/p/articles/mi_m0FDL/is_4_14/ai_n24940334/ http://findarticles.com/p/articles/mi_m0FDL/is_4_14/ai_n24940334/
- ↑ Lack Energy? Maybe It's Your Magnesium Level http:://www.ars.usda.gov/is/AR/archive/may04/energy0504.htm?pf=1 http://www.ars.usda.gov/is/AR/archive/may04/energy0504.htm?pf=1
- ↑ Iron, zinc and magnesium nutrition and athletic performance. http:://grande.nal.usda.gov/ibids/index.php?mode2=detail&origin=ibids_references&therow=268068 http://grande.nal.usda.gov/ibids/index.php?mode2=detail&origin=ibids_references&therow=268068
- ↑ Micronutrients (magnesium, zinc, and copper): are mineral supplements needed for athletes? http:://www.ncbi.nlm.nih.gov/pubmed/7550259 http://www.ncbi.nlm.nih.gov/pubmed/7550259
- ↑ 7.0 7.1 7.2 Linus Pauling Institute - Magnesium http:://lpi.oregonstate.edu/infocenter/minerals/magnesium/ http://lpi.oregonstate.edu/infocenter/minerals/magnesium/
- ↑ Magnesium supplementation and osteoporosis http:://www.ncbi.nlm.nih.gov/pubmed/7770187?dopt=Abstract http://www.ncbi.nlm.nih.gov/pubmed/7770187?dopt=Abstract
- ↑ Magnesium intake from food and supplements is associated with bone mineral density in healthy older white subjects. http:://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=PubMed&dopt=Citation&list_uids=16274367 http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=PubMed&dopt=Citation&list_uids=16274367
- ↑ Magnesium: A Key to Calcium Absorption http:://www.mgwater.com/calmagab.shtml http://www.mgwater.com/calmagab.shtml
- ↑ Magnesium Treatment for Migraines An Inexpensive but Equivocal Treatment http:://neurologicalillness.suite101.com/article.cfm/magnesium_treatment_for_migraines http://neurologicalillness.suite101.com/article.cfm/magnesium_treatment_for_migraines
- ↑ Are Migraine Headaches a Symptom of a Magnesium Deficiency? http:://www.ctds.info/magnesium-migraine.html http://www.ctds.info/magnesium-migraine.html
- ↑ Exertional Heat Cramps: Recovery and Return to Play http:://www.sanfordhealth.org/ClassLibrary/Page/Images/files/ExertionalHeatCramps.pdf http://www.sanfordhealth.org/ClassLibrary/Page/Images/files/ExertionalHeatCramps.pdf
- ↑ University of Maryland Medical Center - Magnesium http:://www.umm.edu/altmed/articles/magnesium-000313.htm http://www.umm.edu/altmed/articles/magnesium-000313.htm
- ↑ 15.0 15.1 Magnesium in biology http:://en.wikipedia.org/wiki/Magnesium_in_biology http://en.wikipedia.org/wiki/Magnesium_in_biology
- ↑ Epsom Salts http:://en.wikipedia.org/wiki/Epsom_salts http://en.wikipedia.org/wiki/Epsom_salts